This invention relates to novel, improved high quality brominated styrenic polymers eminently well suited for use as flame retardants in thermoplastic polymer compositions.
Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and nylon (a.k.a. polyamides). Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Heretofore the art has proffered many processes which are claimed to produce a superior brominated polystyrene. See U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322.
Despite these efforts, previously-known brominated polystyrene flame retardants remain deficient in certain properties which translate into deficient performance of thermoplastic polymer blends in which they are used when the blends are subjected to polymer processing conditions.
For example, prior art brominated polystyrene polymers that have been evaluated for thermal stability have exhibited a 1% weight loss at temperatures less than 336.degree. C. when submitted to Thermogravimetric Analysis (TGA) and, indeed, most have exhibited a 1% weight loss at temperatures around 300.degree. C. Such low thermal stabilities are undesirable, especially under the high temperatures to which flame retarded thermoplastics formulated with such brominated polystyrene polymers are exposed during processing.
Corrosion of metal processing equipment such as melt blenders, extruders, and molding machines, attributable to the release of hydrogen halide under thermal processing conditions is another deficiency of flame retarded thermoplastic polymer blends made using prior brominated polystyrene flame retardants. In the presence of moisture, hydrogen chloride and hydrogen bromide released from the brominated polystyrene in the blend during exposure to the elevated polymer processing temperatures can result in acid formation and consequent metal corrosion.
The bromine content of a brominated polystyrene is the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the aliphatic portion(s) of the polymer, e.g., the polymer backbone or alkyl substitution which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation of aromatic rings in the brominated polystyrene is catalyzed by the Lewis acid catalyst used in producing the brominated styrenic polymer, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the total bromine content, its contribution to the total bromine content is small. Nevertheless, as pointed out in U.S. Pat. No. 5,328,983, ionic impurities in brominated polystyrene may degrade polymer formulations in respect to their ultimate electrical properties, and also may result in corrosion of processing equipment or in the corrosion of metallic parts in their end-use applications.
The chlorine content of brominated polystyrenes is credited to chlorine which, like the bromine, is chiefly part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content. However, chlorinated solvents and/or chlorine-containing catalysts used in the production of the brominated polystyrene may also contribute to the chlorine content of the brominated polystyrene.
The aliphatic halide content of the brominated polystyrene is not desirable as aliphatic halide is not as thermally stable as aromatic halide and, thus, aliphatic halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Aliphatic bromide and chloride are generally referred to by the art and quantified, respectively, as hydrolyzable bromide and hydrolyzable chloride since such halides are easily hydrolyzed as compared to aromatic halides.
To evaluate brominated styrenic polymers for their tendencies to release hydrogen halide under thermal processing conditions, use is made of the method described in U.S. Pat. No. 5,726,252 and referred to therein as the Thermal Stability Test Procedure. In essence, this method indicates the content of halogen atoms in the brominated polystyrene that is not bonded directly to the aromatic rings and, thus, is more readily released from the polymer when at elevated temperature. The Thermal Stability Test is described in greater detail hereinafter.
Apart from whether the halide is present as an aromatic or aliphatic halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
Additionally, it has been demonstrated that prior art processes for the manufacture of brominated polystyrene give rise to significant cleavage of the polymer chain. This cleavage results in the produced brominated polystyrene having an M.sub.w, as measured by Gel Permeation Chromatography, which is significantly lower than the calculated theoretical M.sub.w of the brominated polystyrene. The calculation is based upon the bromine content (wt %) of the brominated polystyrene product and the M.sub.w of the polystyrene reactant at reaction initiation. It is advantageous if the theoretical M.sub.w and the actual M.sub.w of the produced brominated polystyrene are close to each other, given the .+-. margins of error for GPC, since such closeness evidences a paucity of polymer cleavage. The degree of cleavage should be minimized since cleavage results in an increase of alkyl end groups in the brominated polystyrene, which alkyl end groups provide loci for the facile formation of the undesirable hydrolyzable halides discussed above. Conversely, if cross-linking occurs, the molecular weight of the brominated polystyrene is increased, and if not controlled, such cross-linking can result in formation of insoluble residues and/or gelation of the reaction mixture. In addition, viscosity specifications related to end product usage can be disrupted by such undesirable increases in molecular weight.
It would be especially desirable if most if not all of the above-mentioned disadvantages of brominated polystyrenes could be avoided or at least minimized. For example, it would be of considerable advantage, especially for achieving better electrical properties in connection with nylon flame retardant usage, if a more thermally stable brominated styrenic polymer, e.g., brominated polystyrene, could be provided that also has a suitably low ionic bromine content. Another example of a welcome contribution to the art would be the provision of a brominated polystyrene styrene polymer in which the theoretical M.sub.w and the actual M.sub.w of the produced brominated polystyrene are close to each other and in which the content of ionic bromine is sufficiently low for inclusion in polymers used in electrical applications, such as glass-filled nylon polymers.
This invention is deemed to at least minimize, if not overcome, most, if not all, of the above-mentioned disadvantages of brominated polystyrenes.